1. Field of Invention
The present invention is directed to a method for improving the effective total area coverage in a marking process, such as printing, photocopying or the like.
2. Description of Related Art
It is obviously desired to reproduce color images such that the colors in the copy exactly, or at least closely, match the corresponding colors in the original image. Since image input and output devices are often quite different, reproducing an accurate color image often requires some form of color conversion to be applied to the image data before it is output. Systems and methods for converting original input device dependent image signals to output device dependent image signals are known to those skilled in the art. U.S. patents which address such techniques include U.S. Pat. No. 5,077,604 to Kivolowitz et al., issued Dec. 31, 1991; U.S. Pat. No. 5,087,126 to Pochieh, issued Feb. 11, 1992; and U.S. Pat. No. 5,528,386 to Rolleston et al., issued Jun. 8, 1996, each of which is herein incorporated by reference in its entirety. To one extent or another, each of these prior art systems and methods generally involve applying the input device dependent signals (e.g., RGB) to one or another systems of look-up tables by which they are converted to printer or other output device dependent signals (e.g., CMY or CMYK).
Full color revision in digital images is an image processing operation which typically takes place in two steps. First, each pixel in the image is corrected from 3-D input device dependent (e.g., RGB) or device independent (e.g., YCrCb, R′G′B′, XYZ, or L*a*b*) color space to a 3-D output device dependent color space (e.g., R″G″B″ or CMY). Next, a conversion to CMYK takes place, which may include under-color removal (UCR), gray-component replacement (GCR) and linearization processes. Typically, the two color revision steps are accomplished in a single operation using a three-dimensional look-up table carrying out three-dimensional interpolation such as that described in U.S. Pat. No. 5,581,376 to Harrington, which is incorporated by reference herein in its entirety. The color revision portion of the process may also be represented as the combination of two phases: a 3-3 color space revision (e.g., RGB to R′G′B′) followed by a device-dependent color space transformation operation (i.e., R′G′B′ to CMYK for four color devices).
GCR is used in the printing industry to reduce the amount of ink or other marking material deposition in a particular area, which enables faster drying times and lower ink usage and cost. In addition, some level of GCR is used to make shadow details more crisp by taking away some of the cyan, magenta, and yellow in equal amounts and replacing it with black.
In a xerographic technology based printing environment, one component that drives development stability is throughput (i.e., the system or individual component output per unit of time). At high area coverage, the marking system becomes unstable due to inadequate admix time of the developer components. At very low area coverage, large residence times of the toner or other marking material in the housing impact additives present in toner or other marking material, causing the toner or other marking material to lose its ability to hold a charge. Neither is desirable.
Typical strategies for high area coverage include dead cycles where toner is added together with developer and mixed. Then printing is resumed. This solution results in reduced print speed, which is typically undesirable.
At low area coverage, the typical solution is to produce test patches which are developed on a photoreceptor, typically at inter-document zones, and taken away at a cleaner. This maintains a minimum throughput of a particular color to maintain stability. However, since these patches do not form part of the image, this solution typically results in higher costs because of unused toner that is thrown away to keep the marking system stable.